VIBRATIONAL CHARACTERISTICS OF MICROSEISMS AND THEIR APPLICABILITY TO MICROZONING IN A SEDIMENTARY BASIN

In order to study the applicability of microseisms to microzoning of an urban area in a sedimentary basin, characteristics of microseisms were examined through simultaneous observations at ground sites and rock sites in and around the Kyoto Basin, southwest Japan. Time-invariant and site-specific soil amplifications were discussed on the basis of spectral ratios averaged over repeated observations (ground site/rock site): (1) horizontal amplification is larger than vertical one; (2) as the site location changes from the edge of basin to the center, the frequency range with large amplification extends to lower frequency, and (3) at the margin inside the basin, amplification in the direction parallel to the edge of basin is larger than that normal to the edge. Amplification factors thus obtained were nearly the same as those for seismic surface waves from the 1984 west Nagano Prefecture earthquake of M 6.8. Duration of vibration at a ground site during the earthquake was much longer than that at a rock site around the peak frequency of amplification for microseisms. Therefore, amplification factor for microseism can be used as a useful measure to predict long-period vibrational characteristics during earthquakes.